I. Field
The present invention relates generally to communication, and more specifically to techniques for transmitting data in a multiple-input multiple-output (MIMO) communication system.
II. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS spatial channels, where NS≦min {NT, NR}. The MIMO system can provide increased transmission capacity if the NS spatial channels created by the multiple transmit and receive antennas are used for data transmission.
A major challenge in a MIMO system is selecting a suitable rate for data transmission based on channel conditions. A “rate” may indicate a particular data rate or information bit rate, a particular coding scheme, a particular modulation scheme, a particular data packet size, and so on. The goal of the rate selection is to maximize throughput on the NS spatial channels while meeting certain quality objectives, which may be quantified by a particular packet error rate (e.g., 1% PER).
The transmission capacity of a MIMO channel is dependent on the signal-to-noise-and-interference ratios (SNRs) achieved by the NS spatial channels. The SNRs are in turn dependent on the channel conditions. In one conventional MIMO system, a transmitter encodes, modulates, and transmits data in accordance with a rate that is selected based on a model of a static MIMO channel. Good performance can be achieved if the model is accurate and if the MIMO channel is relatively static (i.e., does not change over time). In another conventional MIMO system, a receiver estimates the MIMO channel, selects a suitable rate based on the channel estimates, and sends the selected rate to the transmitter. The transmitter then processes and transmits data in accordance with the selected rate. The performance of this system is dependent on the nature of the MIMO channel and the accuracy of the channel estimates.
For both conventional MIMO systems described above, the transmitter typically processes and transmits each data packet at the rate selected for that data packet. The receiver decodes each data packet transmitted by the transmitter and determines whether the packet is decoded correctly or in error. The receiver may send back an acknowledgment (ACK) if the packet is decoded correctly or a negative acknowledgment (NAK) if the packet is decoded in error. The transmitter may retransmit each data packet decoded in error by the receiver, in its entirety, upon receiving a NAK from the receiver for the packet.
The performance of both MIMO systems described above is highly dependent on the accuracy of the rate selection. If the selected rate for a data packet is too conservative (e.g., because the actual SNR is much better than the SNR estimate), then excessive system resources are expended to transmit the data packet and channel capacity is underutilized. Conversely, if the selected rate for the data packet is too aggressive, then the packet may be decoded in error by the receiver and system resources may be expended to retransmit the data packet. Rate selection for a MIMO system is challenging because of (1) greater complexity in the channel estimation for a MIMO channel and (2) the time-varying and independent nature of the multiple spatial channels of the MIMO channel.
There is therefore a need in the art for techniques to efficiently transmit data in a MIMO system and which do not require accurate rate selection in order to achieve good performance.